What is the difference between a DVD+RW and a DVD-RW.

You can check on this tutorial which will show you some more details about DVD-RW disc and drive;
What is DVD RW Disc and Drive

DVD-RW
It is the re-recordable version of the DVD-R. To use this type of media you will need to buy a DVD-RW recorder. The DVD-RW recorders usually record DVD-R, CD-R and CD-RW media, too. Similarly to what happens to DVD-R, DVD-RW disks can be played in newer commercial DVD players without any problems. Older commercial devices cannot recognize the media, refusing to play the disk. To play a DVD-RW disk, commercial players need the disk to be finished. After finishing the disk, you can only record new data in it by reformatting it, which makes you lose all the recorded data.

DVD+RW

It is the re-recordable version of the DVD+R and everything that was said about the DVD+R is valid for the DVD+RW. Only the newest DVD players play movies recorded in disks using this format. There are recorders on the market that can record both DVD-RW disks and DVD+RW ones. These recorders are called DVD?±RW. Similarly to the DVD-RW, the disk needs to be finished to be played in commercial DVD players, and to record new data in the disk after having finished it requires reformatting it, which makes all the data in it to be erased. DVD+RW recorders are usually capable of reading DVD-RW disks (but not of recording them) and vice-versa.

There you are.